Arsenic

For more information on Arsenic Treatment, please contact:

Linda Fiedler
Technology Assessment Branch
(703) 603-7194
fiedler.linda@epa.gov

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Chemistry and Behavior

Arsenic exists in two distinct crystalline forms known as yellow arsenic and metallic arsenic. Yellow arsenic has a relatively low density (3.9 g/cc), is metastable, extremely volatile, and readily degrades to its metallic form. The low density implies an As₄ structure that is similar to P₄. The metallic form is a tin-white mineral that quickly tarnishes to dark gray. Metallic arsenic crystals have a density of 5.73 g/cc, are very brittle, and are good conductors of heat and poor conductors of electricity. Elemental arsenic is not common in nature.

Arsenic's principal ores are sulfides (As₂S₃, As₄S₄, and FeAsS), and arsenic sulfides are almost invariably found with other metal sulfides. The hydrogen form of arsenic is arsine, which is a poisonous gas. Arsenic also forms oxide compounds. Arsenic trioxide (As₂O₃) is a transparent crystal or white powder that is slightly soluble in water and has a specific gravity of 3.74. Arsenic pentoxide (As₂O₅) is a white amorphous solid that is very soluble in water, forming arsenic acid. It has a specific gravity of 4.32.

The most common forms of arsenic in groundwater are their oxy-anions, arsenite (As⁺³) and arsenate (As⁺⁵). Under moderately reducing conditions, arsenite is the predominant species. In oxygenated water, arsenate is the predominant species. Both anions are capable of adsorbing to various subsurface materials, such as ferric oxides and clay particles. Ferric oxides are particularly important to arsenate fate and transport as ferric oxides are abundant in the subsurface and arsenate strongly adsorbs to these surfaces in acidic to neutral waters. An increase in the pH to an alkaline condition will cause both arsenite and arsenate to desorb. Hence, they can be expected to be very mobile in an alkaline environment. The arsenic oxy-anions are also sensitive to redox conditions, and the speciation differential between them will change with changing redox (Henkel and Polette 1999).

Arsenic also forms organic compounds.

Arsenic Behavior under Sulfate-Reducing Conditions: Beware of the "Danger Zone"
Carol L. Stein, Gannett Fleming, Inc.
EPA Science Forum 2005: Collaborative Science for Environmental Solutions, May 16-18, 2005, Washington, DC.
Contact: Carol L. Stein, clsteinNH@adelphia.net

ATSDR Toxicological Profile for Arsenic
Agency for Toxic Substances and Disease Registry, Aug 2007

Environmental Chemistry of Arsenic
Frankenberger, W.T. (ed.)
Marcel Dekker, New York. ISBN: 0-8247-0676-5, 410 pp, 2002

Field Study of the Fate of Arsenic, Lead, and Zinc at the Ground-Water/ Surface-Water Interface
U.S. EPA, National Risk Management Research Laboratory, Ada, OK.
EPA 600-R-05-161, 91 pp, 2005

Handbook of Elemental Speciation, II: Species in the Environment, Food, Medicine and Occupational Health
R. Cornelis, J. Caruso, H. Crews, and K. Heumann, eds.
John Wiley & Sons, New York. ISBN 0-470-85598-3, 768 pp, 2005.

Covers the speciation of elements from aluminum to zinc, including arsenic, chromium, and mercury.

The Impact of Ground-Water/Surface-Water Interactions on Contaminant Transport with Application to an Arsenic Contaminated Site
Robert Ford, U.S. EPA, National Risk Management Research Laboratory, Ada, OK.
EPA 600-S-05-002, 22 pp, 2005

This document provides a brief overview of the dynamics of chemical processes that govern contaminant transport and speciation during water exchange across the ground-water/surface-water transition zone and presents results from a field study examining the fate of arsenic during ground-water discharge into a shallow lake at a contaminated site.

Impacts of CCA-Treated Wood on Arsenic Concentrations in Soils and Plants
L. Ma, T. Chirenje, and J. Santos.
Florida Center for Solid and Hazardous Waste Management, Report 06-50892, 28 pp, 2006

The examination of soils and plants that have had 44 or more years of ground contact with chromated copper arsenate (CCA) provided a unique opportunity to study long-term migration of CCA constituents at several sites.

Natural Remediation of Arsenic Contaminated Ground Water Associated with Landfill Leachate
Kenneth G. Stollenwerk and John A. Colman.
U.S. Geological Survey Fact Sheet 2004-3057, 4 pp, 2004.

This fact sheet describes results of studies by the U.S. Geological Survey at the Saco Municipal Landfill, Saco, Maine. The source of arsenic in ground water and effects of landfill leachate on arsenic concentration in ground water are described.

Partition Coefficients for Metals in Surface Water, Soil, and Waste
Allison, Jerry D. (HydroGeoLogic, Inc., Herndon, VA); Terry L. Allison (Allison Geoscience Consultants, Inc., Flowery Branch, GA).
Report No: EPA 600-R-05-074, p , July 2005

This report presents metal partition coefficients for the surface water pathway and for the source model used in the multimedia, multi-pathway, multi-receptor exposure and risk assessment (3MRA) technology under development by U.S. EPA. Literature searches, statistical methods, geochemical speciation modeling, and expert judgment were used to provide reasonable estimates of partition coefficients for antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, molybdenum, mercury, methylated mercury, nickel, selenium, silver, thallium, tin, vanadium, and zinc.

Relation of Arsenic, Iron, and Manganese in Ground Water to Aquifer Type, Bedrock Lithogeochemistry, and Land Use in the New England Coastal Basins
J.D Ayotte, M.G. Nielsen, G.R. Robinson, Jr., and R.B. Moore.
U.S. Geological Survey Water Resources Investigations Report 99-4162, 70 pp, 1999
Contact: Joseph Ayotte, jayotte@usgs.gov

Rock-Bound Arsenic Influences Ground Water and Sediment Chemistry Throughout New England
G.R. Robinson Jr. and Joseph D. Ayotte. U.S. Geological Survey, Open-File Report 2007-1119, 18 pp, 2007

Both stream sediment chemistry and the solubility and mobility of arsenic in ground water in bedrock are influenced by host-rock arsenic concentrations. Stream sediment chemistry and the distribution of geologic units are useful parameters to predict the areas of greatest concern for elevated arsenic in ground water and to estimate the likely levels of human exposure to elevated arsenic in drinking water in New England; however, the extreme local variability of arsenic concentrations in ground water from these rock sources indicates that arsenic concentrations in ground water are affected by other factors in addition to arsenic concentrations in rock.

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Last updated on Tuesday, July 1, 2008